Introduction

In the serial number feat, you ask someone to take out a bill with a serial number on it, and hold it so that you cannot see the serial number. You then ask for the total of the 1st and 2nd digits in the serial number (ignoring any letters that may be present). After this, you ask for the total of the 2nd and 3rd digits, followed by the total of the 3rd and 4th digits, and so on, finishing with the total of the 7th and 8th digits. You then ask for the total of the 2nd and 8th digits, as well. Once you have all this information, you're able to give the exact serial number on their bill!

This feat was originated by Royal V. Heath. He simply wrote the totals down as they were given, and worked the serial number out on paper. He was nonetheless able to give the serial number quicker than most people would think possible.

Method

For this feat, you require only a notepad and a pen or pencil. In what follows, I'm assuming a USA bill with an 8-digit serial number is being used. This routine is easily adapted for other bills, as long as they contain an even number of digits.

When they give you the total of the 1st and 2nd digits of the serial number, write them down on a notepad. To the right of that, write the total they give you for the 2nd and 3rd digits of the notepad, and so on. Once you've reached the point where you've asked for the total of the 7th and 8th digits, you should have 7 totals written on your notepad. Finally, ask for the total of the 2nd and 8th digits, and write that to the right of the other numbers.

As an example, let's say the serial number is 37036914, but you don't know this yet. The total you're given for the 1st and 2nd digits would be 10 in this case. When given the totals, including the total of the 2nd and 8th digits, your notepad would read: 10-7-3-9-15-10-5-11

The first step to figuring out the original serial number is to add the 2nd, 4th, 6th and 8th totals together. In our example, this would be 7+9+10+11, which gives us 37. Write this down on the notepad, beneath the original list of totals you made.

The second step is to add the 3rd, 5th and 7th totals together (notice that the 1st total isn't included), and write these beneath the calculation you just made. In our example, you would total 3+15+5=23. You would write 23, just below the 37.

With the results of the two calculations you've just made, subtract the first from the second. Continuing with our example, we perform 37-23=14. Note that, regardless of the serial number, this answer will always be an even number.

What ever answer you received from this calculation, divide this number by 2. This result is the 2nd digit of the serial number! Since 14/2=7, we now know that the second digit of our example serial number is a 7!

Using the knowledge we have of the 2nd digit, it is now a relatively simple matter to work out the full serial number by referring to the totals given earlier by the spectator. Looking at the 7 in the example, and referring back to our 10-7-3-9-15-10-5-11 list of totals, we note that the totals of the 1st and 2nd digits is 10. Obviously, the first digit must be a 3 (10-7=3). So the serial number begins with a 3, followed by 7. Using similar logic, we can work out that the serial number is 37036914!